1. Field of the Invention
The present invention relates to a needle thread restraining device in a thread cutting sewing machine.
2. Prior Art
A sewing machine provided with a thread cutting mechanism is defined as a thread cutting sewing machine. In a conventional thread cutting sewing machine, the sewing threads comprising a needle thread and a bobbin thread is cut by a knife under a needle plate on which a work is placed during the time when a needle rises from the surface of the work and moves to an upper stopping position after a series of sewing operations are completed.
It would be ideal if the needle thread which remains at the needle side after the sewing thread is cut by the thread cutting mechanism, has the necessary and sufficient length so as to form a proper seam at the first stitch of the successive start of the sewing operation. If the length of the remaining needle thread is short, there is a possibility that the needle thread comes out of the needle to thereby stop the sewing operation. On the contrary, if the remaining needle thread is long, the end of the needle thread remains long on the first seam of the successive sewing work, thereby deteriorating the quality of the sewn product, which involves the necessity of manual cutting and removing of the end of the needle thread depending on the specification of the sewn product.
To solve such a drawback, an appropriate resistance involved in feeding the sewing thread is applied to the needle thread for drawing an appropriate amount of the needle thread under the needle plate when the needle cutting mechanism operates. A tension regulator or an auxiliary tension regulator is provided as a means for applying such resistance wherein the tension applied by the tension regulator of the sewing machine is nullified during the time when the thread cutting mechanism operates and accordingly a given tension is applied to the needle thread by the auxiliary tension regulator alone to thereby regulate the amount of the needle thread to be drawn under the needle plate. The auxiliary tension regulator is provided between the tension regulator and a spool serving as the needle thread supplying source for always applying the given tension to the needle thread.
However, according to the needle thread restraining device of the conventional thread cutting sewing machine, since the given tension is always applied to the needle thread which is drawn under the needle plate by the auxiliary tension regulator, the tension applied by the auxiliary tension regulator tends to influence unfavorably upon the work depending on the specification of the work. Concretely, it is very difficult to harmonize the balance of the seams between the needle thread with the bobbin thread relating to the tension to be applied to the needle thread with the appropriate length of the needle thread after the sewing thread is cut, which causes the problem in a sewing mill. Particularly, in case of a thin work, the tension applied by the auxiliary tension regulator influences delicately upon the balance of the seams of the needle thread and the bobbin thread.